Tension-relieving device for stretchable sheet material

ABSTRACT

In apparatus for advancing a knitted fabric or other stretchable web material, preferably toward a length-measuring device, a feed roller is drivingly engageable with the material and there are material guide means so arranged as to cause the travelling material to form a shallow sagging portion. The feed roller is driven by speed-controllable drive means whose speed control is subject to response of a proximity sensor disposed near to but out of contact with the sagging portion of material and responsive to at least one of given lower and upper limits of sag respectively. The speed control operates intermittently to vary the speed of the feed roller between leading and lagging speed magnitudes for substantially tension-free advance of the material.

United States Patent 72] Inventor Erich Kamphausen Rbeydl, Germany [21] Appl. No. 792,254 [22] Filed Jan.2l, 1969 [45] Patented June 29, 1971 [73] Assignee A. Moniorts Monchengladbach, Germany [32] Priority Jan. 20, 1968 {33] Germany [31] P 16 38 082.8

[54] TENSION-RELIEVING DEVICE FOR S'I'RETCHABLE SHEET MATERIAL 8 Claims, 2 Drawing Figs.

[52] US. Cl ,[50] Field of Search [56] References Cited UNITED STATES PATENTS 2,115,737 5/1938 Menschner iililr,

2,707,254 4/1955 Newman 226/30 X 2,877,397 3/1959 Poschner.... 226/42 X 3,240,411 3/1966 Zarleng 226/42 Primary Examiner-Richard A. Schacher An0meys-Cuit M. Avery, Arthur E. Wilfond, Herbert L.

Lerner and Daniel J. Tick ABSTRACT: In apparatus for advancing a knitted fabric or other stretchable web material, preferably toward a lengthmeasuring device, a feed roller is drivingly engageable with the material and there are material guide means so arranged as to cause the travelling material to form a shallow sagging portion. The feed roller is driven by speed-controllable drive COUNTER TENSlON-RELIEYING DEVICE FOR STRETCHABLE SHEET MATERIAL My invention relates to length measuring or other apparatus for handling of stretchable web, sheet or tape material, particularly textile fabrics. In a more particular aspect, the invention concerns a tension-relieving device for stretchable material, particularly knitted textile fabrics, which is equipped with at least one driven feed roller whose effective feeding speed is regulated in response to the amount of sag occurring in an upwardly and downwardly limited shallow sagging portion of the travelling material.

Tension-relieving devices of this general type are required in conjunction with measuring machines by the German calibrating standards issued Apr. 14, 1965, which in Section 77, paragraph 2, second sentence, 77The measuring Zmust be provided with a device (tension-relieving device) which takes care of having the material to be measured supplied to the measuring mechanism in a substantially relaxed condition."

It is known to provide a device for longitudinally measuring the length of textile sheet material in an elongation-free condition, with a pulloff (withdrawal) roller and a pair of feed rollers. In this device, the feeding speed of the material is controlled by regulating the amount of slip between feed roller and the fabric, the regulation being controlled with the aid of an electric contact arranged closely beneath the tangential plane common to the rollers (German Pat. No. 672,479).

This known device is not suitable for use with stretchable material, especially knitted fabrics, because the contact engagement of the feeler contact with the travelling material imposes mechanical stress upon the material, and the regulation of the slip retards the feeding travel of the material.

Further known is a tension-relieving device operating without the necessity of making contact with the material. In this device the feeding speed of the material being measured is regulated by permitting the formation of a long and deep sagging portion which cooperates with two photoelectrical devices whose respective optical paths extend in the longitudinal direction of the travelling sheet of fabric.

A device of the latter type cannot be used for sensitive material on account of the great amount of sag required by the minimum spacing needed between the two photoelectrical systems. Such a large amplitude of sag may cause lateral deflection of the travelling sheet of material as well as the formation of folds, aside from the fact that the volumetric weight of the sagging portion causes the material to be lengthened by its own gravity.

None of the tension-relieving devices known for purposes of the above-described kind affords a satisfactory tension relief without undue trouble in cases where it is desired to apply a length-measuring operation to lightweight and stretchable knitted fabrics sensitive to mechanical stresses, as are increasingly produced in modern textile industries.

It is an object of my invention to afford a satisfactory measuring or similar handling of stretchable material which not only satisfies the exacting requirements of the above-mentioned standards but also minimizes or obviates the abovementioned difficulties and shortcomings encountered with the tension-relieving devices heretofore available.

More specifically, it is an object of my invention to devise a tension-relieving device which secures feeding a material sensitive in the above-mentioned respect, such as knitted fabrics, to a length-measuring unit without encountering mechanical stress, lateral departure from the desired path of travel, formation of holes, or lengthening of the material being measured.

To this end, and in accordance with a feature of my invention, l regulate the feeding and travel speed of the stretchable material entering into the operating range of the measuring device, by varying the rotating speed of the feed roller, and for this purpose have the magnitude of the above-mentioned sag determined between an upper and a lower limit value by proximity-res onsi 'e sensor means which thus operate without contacting the travelling material and respond at least to one of the two limits of sag, preferably at least to the upper limit value. The response of the proximity sensor means is applied to a control system which affects the driving speed of the feed roller, thus regulating the feeding travel in the sense required to maintain the material at the length-measuring locality free of appreciable tension.

By such avoidance of tension due to speed regulation of the feed roller, the occurrence of slippage between feed roller and work material and hence a corresponding retardation in travel is avoided; due to the slight weight of the web portion which forms the shallow sagging loop, any lengthening of the web due to gravity in the sagging portion remains insignificant, and a lateral departure from the desired path is obviated; since further the sensing is effected without contact engagement between sensor and material, no mechanical stress is imposed by the sensing device upon the travelling material.

In order to secure a satisfactory regulation of the feeding speed, the drive for the feed roller according to the invention is preferably provided with clutch or coupling means through which the sensing device, when responding to the upper limit value of sag, is effective to control the feed roller to increase the feeding speed, while switching the feed roller back to lagging speed when the lower limit value is reached.

The switching of the feed roller to lagging speed is preferably also effected under control by the proximity sensor means. However, such switching of the feed roller to lagging speed may also be controlled by a presettable timing mechanism.

According to another feature of the invention, the sensor device is constituted by a distance regulator for maintaining the surface of the sagging portion of material between a given maximum and a given minimum spacing. Such a distance regulator may consist of a known and commercially available device operating on a photoelectrical principle. Also available are ultrasonic devices and other proximity-type sensors capable of performing the desired regulation of spacing without contact engagement with the travelling material.

According to a particularly favorable embodiment of the invention, the measuring device is built into a display-type measuring machine with a forward and reverse drive, a driven pulloff or withdrawal roller being provided downstream of the measuring device seen in the forward travel direction of the web material. According to the invention, the rotating speed of the withdrawal roller is controlled by second sensor means which respond to a second shallow sagging portion of the material. The withdrawal roller is connected with its drive through transmission means which are under control by the second sensor so that when this sensor responds to the upper or lower limit value of sag, it will switch the withdrawal roller to leading or lagging speed.

According to a further feature of the invention, a satisfactory tension relief in both travel directions of the web material is secured by changing the switching point at which the sensor system changes between leading and lagging speeds, this change in setting being controlled by a switching device coupled with the drive controls that selectively set the machine for forward and reverse run of the web material. As a result, the point of response of each sensing system is automatically 7 reset for reverse travel of the web material so as to differ from the critical triggering point obtaining during forward travel.

In some cases apparatus according to the invention may exhibit an undesirably high switching frequency. To avoid this, and in accordance with still another feature of the invention, 1 provide the system with means which, during forward travel of the web material, delay the moment at which the feed roller is The above-mentioned objects, advantages and features of my invention, said features being set forth with particularity in the claims annexed hereto, will be apparent from, and will be described in, the following with reference to an embodiment of a display-type measuring machine for textile web material illustrated by way of example on the accompanying drawing in which:

PK]. 1 shows schematically a lateral elevation of the machine and FIG. 2 is a schematic diagram of the appertaining drive controls.

The illustrated machine is equipped with a measuring roller 1 and a conventional measuring head 1A having a measuring wheel 1a (FIG. 2), a printing band of rubber, and a counting mechanism lb (FIG. 2). The web material is taken from a supply roll SR and, after passing through the measuring means of the machine, is rolled up on a takeup roll TR. The travel movement of the web material from the supply SR to the takeup TR is effected with the aid of a feed roller 2 and a pulloff or withdrawal roller 3, both driven from the measuring roller 1 through controllable transmissions. The path of the web material through the machine is further determined by auxiliary guide rollers which freely rotate about their respective axes due to their engagement with the travelling web material. Some of the guide rollers are denoted in FIG, 1 by 7, 8, 9, l2 and 13. The roller 12 isjournaled on a slider 14 which can be shifted upwardly or downwardly, such as to the psitlon of roller 12 shown by broken lines at 12'. Between rollers 12 and 13 the material 6 is visible from the right of the machine as shown in H6. 1, thus displaying the material as it is being measured and is imprinted along its edge with lndlcla oilength.

A sensing device 4 is located between the feed roller 2 and the measuring roller l, and another sensin device 5 is located between the measuring roller 1 and the withdrawal roller 3. Ea h of the two sensing devices 4 and 5 is mounted above the web mat rial 6 be ng measured. Both sensing devices 4 and 5 are of the proximity type; that is, they operate without bodily engaging the material 6 and may be constituted by conventional optical measuring devi es based on photoelectronlc or ultrasonic principles However, other measuring or sensing devices that operate without conta t can also be used for the purpos of this invention.

The rate of travel or the material 6 is primarily determined by the rotary speed of the measuring roller 1. Between the feed roller 2 and a subsequent guide roller 7 h tr v lling material forms a shallow sagg ng portion bel w the sensing device t. An ther shallow sag occurs bel w h s n i g device 5 between the two guide rollers t3 and 9 preceding the withdrawal roller- 3- By shallow agging portion is meant a loo wherein the spacing of the lowest po n fr m a plan ang ntial to both of the two rollers b tween wh h the l p is suspended, varies between about 5 mm and about 80 mm, the amount of sag being dependent upon the m t al pa ing of the two rollers between which the loop is s p n ed, a well as upon the properties o the ma erial 6, s ch as i s st ength and volumetric wcighb Each of the two sensing devices 4 and 5 has an upper and a lower switching or trigger point corre ponding respec iv ly to a minimum and maximum spa ing rom the surface f t e material a being measured, and hence to respe tive maxim m and minimum limits of in p actice, the s it h g d stance distance between the upper and the lower swi ching points corresponds to a spacing difference of about l5 from the surface cfthe material 6.

Referring now to the schematic diagram or the dri controls shown for esplanatory purposes and by way of example in l lfiit 2 of the drawing, it will be remcmb that th feed roller is driven from the measuring rol 1 and tha the transmission between these two rollers contains a coupling or similar control means which permits swit hing t e fee roller to a leading or lagging sa es relati e to he ate web tra el speed that corresponds to the peripheral spee of he measuring roller 1. In other words, the transmission between rollers 1 and 2 includes a suitable speed change mechanism that permits to selectively drive the feed roller 2 at a slightly faster speed and at a slightly lower speed respectively than'corresponds to the accurate web travel speed desired. Accordingly, FIG. 2 shows the feed roller 2 driven from a sheave 1c on the shaft of the measuring roller 1 which receives its driving power from an electric motor M. The sheave 1c is connected with a belt sheave 21 by an endless belt or chain 20. The sheave 21 is mounted on the input shaft of a speed change mechanism 22 whose speed control member is diagrammatically represented by a lever that can be tilted to the fast or slow position with the aid of respective electromagnet coils F2 and S2. The coils F2 and S2 are energized from a suitable current source 24 under control by respective time delay relays 25 which in turn are energized by switching pulses through respective amplifiers 26 from the outputs of a flip-flop circuit 27. The flip-flop circuit 27 is controlled by the output of a controllable amplifier stage 28 whose input circuit is connected to the sensor 4. The control of the amplifier 28 is offectcd by means of a control circuit 29 for the purpose of selectively setting the point of response at which the sensor will trigger the flip-flop 27 to issue an energizing pulse to the proper relay 25 which then supplies a switching pulse to the coil F2 or S2 in order to shift the speed change mechanism 22 from its previously obtaining state to the other state, i.e. from faster to slower or conversely from slower to faster rotation of the feed roller 2.

Analogously, the withdrawal roller is driven from the sheave 10 on the shaft of the measuring roller 1, or from a corresponding second sheave on the same shaft, through an endless belt 30 which engages a sheave 31 on the input shaft of a second speed change coupling or mechanism 32 whose output shaft drives the withdrawal roller 3. The control member 33 is positlonally controlled by two elcctromagnet coils F3 and S3. Coil F3 is connected to a current source 34 under control by a time delay relay 35 which receives energizing pulses from an amplifier 36. This amplifier is shown connected to only one of the two outputs of a flip-flop 37 which receives controlling signals from a controllable amplifier stage 38 whose input circuit is connected to the second sensor 5 and which has a control circuit 39 for selectively setting the point of response. While the second control coil 53 of the speed change transmission for the withdrawal roller 3 may be designed in the same manner as the one described above with reference to the coil 52, the example shown in FlG. 2 has the coil S3 energized from a current source 40 under control by a timer 4!, This will be explained hereinafter.

The above-described setting of the amplifier stage 28 is controlled by having its control circuit 29 energized from a source 50 of adjustable voltage under control by an auxiliary contact d2 of a contactor or switch l C whose main contacts 43 and 44, when closed, connect the drive motor M of the measuring roller 1 to the current supply with the polarity required for forward travel of the web materiel. Analogously, the control circuit 39 of the amplifier 35% is energized from the source 50 of adjustable voltage through an auxiliary contact 45 of a contactor or switch RC whose main contacts 46 and 47 are closed when the motor M is to run the machinery in the reverse direction.

when the machine is in operation, the degree of sag in the slightly sagging portion of the web material continually varies between the upper and lower limit values thus causing the sow sor i to switch the feed roller 2 alternately t leading and lagging speed, When the surface of the web material approaches the sensor 4 up to the upper limit value, the feed roller g is switched by the controllable transmission means to run at th faster speed. Consequently, the web surface now moves away from the, upper limit value so that the degree of 5 1; insrbssesi When me the lower limit value is reached, the sensor it switches the driving transmission of feed roller 2 to the lagging speed so that the web surface again commences to approach the upper limit value. By virtue of this intermittent control of the feeding speed between two limit values, the loop of material between the rollers 2 and 7 is kept within the desired narrow limits of sag.

The sensor 5, operating in the same manner as the sensor 4, is not necessarily required for calibratable measuring operation during forward travel of the web material, but offers the advantage of winding the measured materialupon the takeup roller TR with a regulated tension. The control system which connects the sensor 5 with the two-speed transmission between the measuring roller 1 and the withdrawal roller 3 has the effect that the withdrawal roller 3 is switched to'lagging speed when the upper limit value of sag is reached and is switched to leading speed when the lower limit value is reached.

As mentioned above, the occurring frequency of switching between faster and slower speeds may be found excessive in which case it is desirable to provide means for delaying the switching of the feed roller 2 from the faster to the slower speed. In an analogous manner, the switching of the withdrawal roller toward leading (faster) speed can be delayed. Applicable for the purpose of such delay are timedelay relays or any other suitable timing circuits or mechanisms. In FIG. 2, therefore, the relays 25 and 35 are preferably of the time-delay type. A corresponding time delay is embodied in the timer ll.

The foregoing description of the control operation relates to the forward travel of the web material 6 to be measured. In display-type machines which permit reversing the rotational direction of all driven rollers, it is necessary to provide for reverse travel of the web material. For such purposes the second sensor 5 is a prerequisite for achieving a sufficient relief of tension in the web material during reverse travel.

However, simply providing twotension-relieving devices upstream and downstream of the measuring roller 1 with respect to the web travel does not yet satisfy the requirement for calibratable measuring performance during reverse travel. For that reason, the switching points of the sensing devices 4 and 5 for leading and lagging speed are changed in order to adapt these points to forward and reverse travel respectively. This change in adjustment is effected by switching means that are coupled with the control device for reverse travel of the material. As described, the travel direction depends upon the running direction of the motor M which is changeable by actuating one or the other of the contactors FC and RC, and such actuation also effects a proper setting of the amplifying stages 28 and 38 in the two sensing and drive control systems for the feed roller '2 and the withdrawal roller 3. During reverse travel of the web material 6, therefore, the sensor 5 is now active to act as the tension-relieving device which precedes the measuring roller l in the travel direction, whereas the roller 2 now operates as the withdrawal roller and the sensor 4 takes care of regulating the tension in the material behind the measuring roller 1 as it iswound onto the takeup roll now constituted by the roller SR.

The switching points of the sensing systems controlled by the sensors 4 and 5, as far as the respective upper limit values are concerned, must be responded to at a moment prior to the one at which the web material would form a straight tangent between the rollers 2 and 7 or 8 and 9, so that the occurrence of longitudinal stress and consequently elongation of the web material 6 is prevented.

In the above-described embodiment, the switching of the feed roller 2 from leading to lagging speed is primarily controlled by the sensor 4. However, such control may also be effected with the aid of a presettable timing mechanism in which case the sensing system exhibits only one switching point and responds only, to the upper limit value of sag. The timing mechanism is to be preset in dependence upon the travel speed of the web material in such a manner that the surface of the web material has at least approximately attained the lower limit value within the duration of the faster, leading speed and hence before the timing mechanism switches the feed roller 2 to the lower, lagging speed. An analogous modification is also applicable to the second tension-relieving device located downstream of the measuring roller 1 and related to the withdrawal roller and the sensor 5. The latter modification is exemplified by the timer 41 in FIG. 2 which is assumed to be set manually in accordance with the web travel speed observed on a speed indicator, although it will be understood that an automatic setting of the timer in response to the travel speed or changes thereof is likewise applicable. It should also be understood that it is preferred to employ the same type of control system for both sensors 4 and 5, the one shown for sensor 4 being considered more advantageous in most cases.

The effect of gravity or inherent weight upon the sagging portion of the web material is negligible, on account of the shallowness of the sag in devices according to the invention. However, with certain types of material an appreciable gravity effect is observed. Such effect can partially and satisfactorily be compensated by applying a controllable jet or flow of air to the bottom side of the travelling web with the aid of air nozzles 10 and 11 mounted beneath the path of the web material 6 in the vicinity of each sagging portion. A complete weight compensation of the material in the sagging portion, however, must not take place because the formation of a sag required for regulating purposes according to the invention is predicated upon a minimum of loop weight.

For the reasons explained, apparatus according to the invention afford the possibility of applying a tension relief to stretchable material, particularly to knitted textile fabrics, to an extent heretofore not attained and in a particularly uniform and satisfactory manner.

It will be understood that the supply and takeup devices such as the rolls SR and TR with the appertaining supports are conventional and may be substituted by any other suitable supply and takeup means respectively. To those skilled in the art it will further be obvious upon a study of this disclosure that my invention permits of various other modifications and hence may be given embodiments other than illustrated and described herein, without departing from the essential features of my invention and within the scope of the claims annexed hereto.

1 claim:

1. With apparatus for advancing a knitted fabric or other stretchable web material, comprising a roller drivingly engageable with the material for imparting advancing travel to the material, material guide means spaced from said roller in the travel direction and arrangedto cause formation of a shallow sagging portion of the travelling material, the combination of means for relieving tension in said travelling material comprising controllable drive means connected to'said roller, speed control means connected to said drive means and having a proximity sensor arranged for out-of-contact response to said sagging portion of material and responsive to at least one of given lower and upper limits of said respectively, said drive means being speed controlled by said sensor response to regulate thespeed of said roller for substantially tension-free advance of the material, said drive means comprising a speed change transmission drivingly connected with said roller and having two selective speeds above and below respectively the desired travel speed of the material, said speed change transmission being set by said speed control means to the leading speed when said sensor responds to said upper limit of sag and being set to the lagging speed when said lower limit of sag is reached, said speed control means comprising a presettable timer which is connected to said speed change transmission for setting it to said lagging speed.

2. ln apparatus according to claim 1, said speed change transmission having two speed-setting means which are both in connection with said sensor so that said setting to lagging speed is also controlled by response of said sensor.

3. in apparatus according to claim 1, said proximity sensor being responsive to given respective maximum and minimum distances from the surface of the web material in said sagging portion, whereby said speed control means operate substantially as a distance regulator.

4. With apparatusfor measuring the length of knitted fabric or other stretchable web material having measuring means, a feed roller drivingly engageable with the material for passing the material to said measuring means, and material guide means arranged to cause formation of a shallow sagging portion of the travelling material, the combination of means for relieving tension in said travelling material comprising controllable drive means connected to said feed roller, speed control means connected to said drive means and having a proximity sensor arranged for out-of-contact response to said sagging portion of material and being responsive to at least one of given lower and upper limits of sag respectively, said drive control means being intermittently controllable by said sensor response to vary the speed of said roller between leading and lagging speed magnitudes for substantially tensionfree advance of the material to said measuring means, said measuring means comprising a measuring roller engageable with the web material downstream of said feed roller and having a main drive, said drive means for said feed roller being connected with said main drive, and said drive control means being connected to said feed roller drive so as to vary the feed roller peripheral speed between respective values above and below the peripheral speed of said measuring roller, and further including a withdrawal roller engageable with the web material and arranged downstream of said measuring roller, further drive means connecting said main drive with said withdrawal roller, and further speed control means for varying the peripheral speed of said withdrawal roller between respective values above and below the peripheral speed of said measuring roller, means for forming a second shallow sagging portion of the web material between said measuring roller and said withdrawal roller, said further speed control means having a proximity sensor responsive to the sag of said web portion.

5. In apparatus according to claim 4, said speed control means comprising time delay means for delaying the switching of said feed roller to the lagging speed and the switching of said withdrawal roller to the leading speed during forward travel of the web material.

6. ln apparatus according to claim 4, said main drive comprising selective reversing means for causing the web material to travel in the forward and reverse direction respectively, and switching means connected to said speed control means for shifting the points of response of said sensors at which said rollers switch between leading and lagging speeds, said switching means being coupled with said reversing means so as to be effective when said main drive is switched to reverse travel of the web material.

7. Apparatus according to claim 4, comprising two blow devices mounted at respective localities beneath said two sagging portions of the web material for partially compensating the weight of said web portions.

8. With apparatus for advancing a knitted fabric or other stretchable web material, comprising a roller drivingly engageable with the material for imparting advancing travel to the material, material guide means spaced from said roller in the travel direction and arranged to cause formation of a shallow sagging portion of the travelling material, the combination of means for relieving tension in said travelling material comprising controllable drive means connected to said roller, speed control means connected to said drive means and having a proximity sensor arranged for out-of-contact response to said sagging portion of material and responsive to at least one of given lower and upper limits of sag respectively, said drive means being speed controlled by said sensor response to regulate the speed of said roller for substantially tension-free advance of the material, and a blow device mounted beneath the sagging portion of the web material for partially compensating the weight of said web portion. 

1. With apparatus for advancing a knitted fabric or other stretchable web material, comprising a roller drivingly engageable with the material for imparting advancing travel to the material, material guide means spaced from said roller in the travel direction and arranged to cause formation of a shallow sagging portion of the travelling material, the combination of means for relieving tension in said travelling material comprising controllable drive means connected to said roller, speed control means connected to said drive means and having a proximity sensor arranged for out-of-contact response to said sagging portion of material and responsive to at least one of given lower and upper limits of said respectively, said drive means being speed controlled by said sensor response to regulate the speed of said roller for substantially tension-free advance of the material, said drive means comprising a speed change transmission drivingly connected with said roller and having two selective speeds above and below respectively the desired travel speed of the material, said speed change transmission being set by said speed control means to the leading speed when said sensor responds to said upper limit of sag and being set to the lagging speed when said lower limit of sag is reached, said speed control means comprising a presettable timer which is connected to said speed change transmission for setting it to said lagging speed.
 2. In apparatus according to claim 1, said speed change transmission having two speed-setting means which are both in connection with said sensor so that said setting to lagging speed is also controlled by response of said sensor.
 3. In apparatus according to claim 1, said proximity sensor being responsive to given respective maximum and minimum distances from the surface of the web material in said sagging portion, whereby said speed control means operate substantially as a distance regulator.
 4. With apparatus for measuring the length of knitted fabric or other stretchable web material having measuring means, a feed roller drivingly engageable with the material for passing the material to said measuring means, and material guide means arranged to cause formation of a shallow sagging portion of the travelling material, the combination of means for relieving tension in said travelling material comprising controllable drive means connected to said feed roller, speed control means connected to said drive means and having a proximity sensor arranged for out-of-contact response to said sagging portion of material and being responsive to at least one of given lower and upper limits of sag respectively, said drive control means being intermittently controllable by said sensor response to vary the speed of said roller between leading and lagging speed magnitudes for substantially tension-free advance of the material to said measuring means, said measuring means comprising a measuring roller engageable with the web material downstream of said feed roller and having a main drive, said drive means for said feed roller being connected with said main drive, and said drive control means being connected to said feed roller drive So as to vary the feed roller peripheral speed between respective values above and below the peripheral speed of said measuring roller, and further including a withdrawal roller engageable with the web material and arranged downstream of said measuring roller, further drive means connecting said main drive with said withdrawal roller, and further speed control means for varying the peripheral speed of said withdrawal roller between respective values above and below the peripheral speed of said measuring roller, means for forming a second shallow sagging portion of the web material between said measuring roller and said withdrawal roller, said further speed control means having a proximity sensor responsive to the sag of said web portion.
 5. In apparatus according to claim 4, said speed control means comprising time delay means for delaying the switching of said feed roller to the lagging speed and the switching of said withdrawal roller to the leading speed during forward travel of the web material.
 6. In apparatus according to claim 4, said main drive comprising selective reversing means for causing the web material to travel in the forward and reverse direction respectively, and switching means connected to said speed control means for shifting the points of response of said sensors at which said rollers switch between leading and lagging speeds, said switching means being coupled with said reversing means so as to be effective when said main drive is switched to reverse travel of the web material.
 7. Apparatus according to claim 4, comprising two blow devices mounted at respective localities beneath said two sagging portions of the web material for partially compensating the weight of said web portions.
 8. With apparatus for advancing a knitted fabric or other stretchable web material, comprising a roller drivingly engageable with the material for imparting advancing travel to the material, material guide means spaced from said roller in the travel direction and arranged to cause formation of a shallow sagging portion of the travelling material, the combination of means for relieving tension in said travelling material comprising controllable drive means connected to said roller, speed control means connected to said drive means and having a proximity sensor arranged for out-of-contact response to said sagging portion of material and responsive to at least one of given lower and upper limits of sag respectively, said drive means being speed controlled by said sensor response to regulate the speed of said roller for substantially tension-free advance of the material, and a blow device mounted beneath the sagging portion of the web material for partially compensating the weight of said web portion. 